Antenna



Dec. 16, 1952 A. ALFORD ANTENNA Filed Jan. 13, 1949 'FIGI I V l 3 D 34 I II 3 30 1.2 E a /.0 z I I I q l l I A E 0.08 o./o 'o.l2 0.74 0J6 0.78 E

LOOP DIAMETER IN WAVE LENGTHS INVENTOR ANDREW/U. FORD BY. I" lsq ATTORNEY Patented Dec. 16, 1952 UNITED STATES PATENT OFFICE 15 Claims.

This invention relates to antennas for ultrahigh frequencies and more particularly to antennas for broadcasting ultrahigh frequency horizontally polarized waves.

This invention makes use of a number of small loops shunted across a balanced transmission line arranged so that a large number of loops may be cophasally energized thereby obtaining a large concentration or radiant power in a plane in which the radiation is distributed in a substantially circular pattern.

One object of this invention is a simple antenna with low windage that could be installed on tall metal masts and energized with one feeder.

Another object of my invention is an antenna that has power gain with respect to antennas of prior art so that two antennas of the invention could be used to produce the same concentration of radiated field as is normally produced by four antennas of the prior art. This result is particularly desirable where a large power gain is desired without employing the customary stacked array consisting of a number of antennas requiring individual means for phasing with the necessary fittings, and transformers must be provided.

Still another object of my invention is an antenna which radiates horizontally polarized waves substantially equally in all horizontal directions when installed on a metal pole.

A further object of my invention is an antenna which is so arranged that it may be conveniently fed. by a coaxial transmission line having a characteristic impedance of the order of 70 ohms.

These and other objects of this invention will more clearly appear from the appended claims and the description which with the aid of Fig. 1 explains certain phases of the invention. Fig. 2 is a graph showing certain factors affecting the operation of the antenna of the invention,

and Figs. 3 and 4 show two embodiments.

l and 2 in Fig. 1 are conductors of a balanced transmission line that is shunted by a plurality of conductors bent into loops such as 3, 4, 5. At one end the conductors l, 2 are connected to a balanced ultrahigh. frequency power source 5. The opposite end of the transmission line is short-circuited by a bar 1. When the number of loops 3, 4, 5, etc. per free space wavelength measured along conductors I, 2 is of the order of IO and the diameter of each loop is of the order of .115 wavelength, then the loops 3, 4, 5, etc. will substantially increase the phase velocity at which waves are propagated along conductors I, 2.

The waves propagated along conductors l, 2 are reflected from short-circuiting bar 1, whereby standing waves are produced along conductors I, 2. The difference of potentials between conductors I, 2 is distributed as indicated by curve 8 in Fig. 1. The difference of potential is substantially zero at the short-circuiting bar 7, reaches a first maximum at 9, the first minimum at In and a second maximum at H. The distance between the first minimum l0 and the short-circuiting bar 1 is substantially equal to half of the free space wavelength at the frequency of source 6 when all the loops are removed. When the loops are shunted across the transmission line I, 2 the distance between 9 and l is greater than a free space half wavelength by a factor K which is a function of the diameter D of each loop and of the spacing L between the consecutive loops.

Fig. 2 shows how the value of K is affected when the diameter D is varied while the ratio L/D is kept constant. In Fig. 2 the diameter D in free space wavelength is plotted along the abscissa and the value of K is the ordinate. Curve l4 indicates the values of K which corresponds to different values of D when the value of L/D is 1.79. Curve [5 shows the value of K which corresponds to different values of D when L/D=.895. Curve [6 shows the value of K for different values of D when the loops have been replaced by a continuous cylindrical sheet of the same diameter. This case corresponds to a very large number of loops adjacent to and in contact with each other, so that the spacing L between them is substantially zero and L/D=0.

It may be seen from Fig. 2 that when the diameter D of each ring is .11 wavelength and the spacing L between the consecutive loops is such that L/D=.895 the distance between the first minimum 10 and the short-circuited bar 1 is approximately two free space half wavelengths. It is found that under these conditions the potentials applied to all loops that are connected to conductors I, 2 between 7 and I!) in Fig. 1 are substantially in the same relative phase except in the immediate vicinity of the minimum H].

The principles explained in connection with Figs. 1 and 2 are utilized in the construction of the antenna of Fig. 3. In the figure, conductors 2|, 22 and loops 23, 24, 25 correspond to conductors I, 2 and loops 3, 4, 5 in Fig. 1. Movable short-circuiting bar 26 is used to reflect waves originated by ultrahigh frequency source 21 which supplies ultrahigh frequency power through coaxial transmission line 28 to conductors 2|, 22. The inner conductor 29 of the coaxial transmission line 28 is connected to condoctor 22. The outer conductor of line 28 is connected to conductor 2|. Movable short-circuiting bar 3! connected across the coaxial transmission line and a tube 39 attached to the lower end of conductor 22, provides a means for improving the impedance match between the balanced transmission line 2|, 22 and the coaxial transmissionlinelfi .while at.the.=same.time insuring, nearly equal and oppositevoltages along conductors 2 I, 22.

Curve 32 indicates the distribution of the difierence of potential existing between conductors 2|, 22. At a point 33 the difference of potential is maximum. data as is shown in Fig. 2 the distance between 26 and 31 may be selected sothat-Siiris located approximately halfway between the..short,-.-circuiting bars 25 and 3|. The final adjustments may be made by moving short-circuiting bar 26 toward or away from 3|. 'When-short-circuiting bar' 26 is in such apositionthat the first minimum-is at or near SLLthe-imp'edance presented to the coaxial transmission line 28 will be nearly a pure'resistance. 'The value of this resistance depends on the distance between ends 34 of the coaxial transmission line sections 28a, 3% and short-circu'iting bar 3!. By moving 3: toward 34- and then by correspondingly adjusting the position of 26 the value-of the impedance presented'at theend 'of the coaxial transmission "line 28may be reduced. By'moving 3! away from 34 and by correspondingly adjusting '26, the value of impedance 'presented :toI line 28 rnay be increased. Such-adjustments may be used to achieve either a goodmatch between line 28 and the-balanced line'2l, 22 or toprovide a convenient value of. impedance for matching the concentric line 28- with:known means such as a quarter-wave transformer.

Thephase of the: difference .ofrpotential between conduetorsl l ,2 22..is substantially constant at the pointswhere 'loopst23; 24,'25,'etc. are connected. The currentswhich'flow in the loopsare thereforesubstantially.in the samerelative phase with :each other a so. that the'loops' are effectively arranged in astacked-cophasal array. This fact results .in a substantial concentration of the pow'er radiatedby the :loops in the direction of the plane: passing through the halfway point .along1conductors2L-22 nd .at'right angles to "the longf'dimension of said conductors 2|, 22. The greater isthetotaldistance or span between the-.end' loops r35; 36; 'the greater is the concentrationiof 'energyin-the' direction of said plane at right angles to the conductors 2|, 22. Since 'thetotalspanbetweenthe endloops that can be cophasally energized maybe increased only when thedistance :between 26 and the first minimum isincreasedyit is desirable to select those combinations of loop-diameter D and loop-spam mg L which result in a large Value of K. This -maybe-done :with the aid of such curves as are shown in Fig. 2. Thus, for example, by selecting loop diameter of .11 wavelength and by spacing these loops so that L/D=.895 it is possible to feed .cophasally with one feeder a stack of'loops having a 'total span approximately .8 wavelength.

The loops 23, 24,25, etc: may be-conveniently supported by a metal pole- Ill at neutral points such as 3% along'loop 23. Neutralpoint 38 is "equidistant from conductors 2!, 22 as measured along conductors of loop '23. Since the potentialat a neutral point'such as 38a is substantially; -zero; now-distortion of radiation. pattern 3 60011175 whenthe-neutralpoints of each loop..are

With the .aid .of/such.

apole .31. wheresaid conductors may be. clamped to the pole r'at points such; as 39, 3.9a.

A still greater concentration of radiant power may be obtained with the arrangement of Fig. 4

which shows another embodiment of my inven- ."tion. In this figure, 40 is a source of ultrahigh frequencypower used to supply conductors 4!,

""42"through coaxial transmission line 53 and an "concentric transmission line 43.

auxiliary balanced feeder 44, 45. Loops such as 46, 41, 48, etc. are shunted across conductors 4|,

"@2011" both sides of the auxiliary feeder M, 15.

Movable short-circuiting bars 49, 59 are shunted across. conductors ll; 42 near their ends.

The reflection of waves ati'short-circuitingbars "49, 50 resultsin a difference of potential between conductors M, 42 which is distributed-as'shown by curve 5!. some of theloops; e: g. "'46,.are

.between' 'shortecircuiting bar 49 and' the' first potential minimum 52, and others, suchas '41, 48,

are shunted across conductors 4|; 42 between short-oircuitingcbar 50 and the potential-minimum 53. 'Preferablyno loopsare; provided between minima 52, 53. ,r'This arrangement provides, therefore, two-asets of loops; the loops in each setbeing energized in the same relative phase with each other.

When the minim 52, 53 are at substantially equal distancesfrom the auxiliary-feeder'M, 45, the loops --in the aupper-and lower sets will be energized in the same phase.

Theloops inpthe two sets will be-energized in the :same-phase'when Ithe distances between short-circuiting bars-49;-50 and feeder- .4-; 45' are :less than the distance from"49-,- 50* to the-nearest -minima"52,-53 so'that these: minima .do not ap- -peanalong conductors. M, '42 but. along'feeders M, t5. The;.,distance.-between 69 andfeeder-N,

45 is preferably 'macleequal to the distance between 50 and 44, 45. Furthermoreyboth these distances are preferably so chosen that there is a-ininimumr in the'neighborhood ofendtt of This proportioning allows a (better match between coaxial transmission line-43' and the transmission line Al -42. A movable short-circuiting bar 55 across 34,-45-provides an additional means for making impedance adjustments while performing its main function of insuring substantially equal and opposite voltages on feeders 44, 45 and, therefore, also on conductors 4|, 42.

The loops may be clamped directly to the metal pole 56 or'to an auxiliary metal channel 5'! bolted to the-pole at neutral'points such as 51a. The ends of conductors M, 42, may be extended beyond the movable short-circuitingbars d9, 59

and clamped to the'metal pole 56-at 58, 59 to provide for additional bracing of the structure. The current flowing in a loop is maximum at .the neutral point but gradually decreases toward the ends of thezloop. The minimum value of our rent. is at the points where the loop is connected to conductors-4|, 42. The radiation pattern of such a'loop int-he plane of the loop is therefore not quite an exact circle but an oval with maxima along'the diameter which'passes through the neutral point '51-, and? the" minima in directions approximately at right angles to saiddiameter.

Asthe size of the loop is decreased the radiation pattern approaches a perfect circle.

The. radiation resistance of each loop varies with the loop diameter. This radiation resistance of a loop consists of two parts; the radiation resistance proper and the mutual radiation resistance. For two closely spaced loops the mutual radiation resistance is approximately equal to the radiation resistance proper so that each loop has a total radiation resistance equal to about twice the-radiation resistance it would have if it were alone. For a number of closely spaced loops the total radiation resistance of each loop is several times as great. When loops are decreased in size and are at the same time separated by greater spaces the radiation resistance of each loop decreases very rapidly until it becomes comparable with the high frequency resistance of the loop conductors. When these two resistances become comparable substantial portion of the power is lost in heating loop conductors. This undesirable condition is usually reached with loops smaller than .07 wavelength in diameter. There are therefore both lower and upper practical limits of the size of loops that may be efficiently used in antennas of Figs. 3 and 4.

' Furthermore, as may be seen from Fig. 2, loops having diameters greater than .17 of a wavelength must be closely spaced requiring a large number of loops per unit length in order that a span ofcophasally energized loops may be greater than .7'of free space wavelength. Loops, whose diameters are less than .07)\, are ineflicient and require high currents. The optimum size of loops is between these two limits in the neighborhood of .115 wavelength.

This optimum loop size is affected to some degree by certain other factors and more particularly by the shape of the cross section of the loop conductors and by the characteristic impedance of the transmission line M, 42 between the loops.

As the, cross section of the individual loop conductor is reduced, the reactance of each loop is increased requiring a smaller spacing between loops. An increase in cross section of loop conductors has an opposite effect, requiring an increase in spacing between loops.

A decrease in the characteristic impedance of the transmission line 4!, 42 requires a proportional decrease in reactance of individual loops or a closer spacing between them. With loops of the order of .115 wavelength in diameter spacing of the order of .10 wavelength is convenient. With loops'of smaller diameter smaller spacing may be desirable in order to avoid substantial portion of the power being radiated by the transmission line in the form of vertically polarized radiation which under ordinary circumstances is essentially lost so far as useful effects are concerned.

Curves l4, l5, I6 in Fig. 2 are given for typical proportions of loops and of transmission line conductors. These curves are based on data obtained with loops made of metal bands of the order of A of a free space wavelength wide and about of a wavelength thick. The transmission line conductors were made of metal strips & wavelength wide and about wavelength thick and spaced about wavelength;

between the nearest edges.

' When the loops are not formed into circles but into other shapes, e. g. an ellipse, a rectangle, etc.-all shapes being covered by the word loop as used in this specification--the operation of the antennas of Figs. 3 and 4 is substantially the same as above described provided that the loops space wavelengths.

The spacing of the transmission line 2|, 22 in Fig. 3 and 4 I, 42 in Fig. 4 is not critical, but should be of the order of a fraction of a wavelength.

It will be clear from the foregoing that broad casting antennas of the present invention can be manufactured in a shop and then installed'on a mast without requiring the soldering of many joints, and that the finished antenna will have low windage and the required qualities as a radiator.

This application contains the subject matter I set forth in my previously filed application, Serial Number 644,519, filed January 31, 1946, now abandoned.

Having now described my invention, I claim:

1. In an antenna, a transmission line, a plurality of more than two loops shunted across said line and means for energizing said line with ultrahigh frequency band of radio waves, said loops being spaced sufficiently close to one another and dimensioned for such a size whereby said loops are contained in a space not more than one-half of an apparent free wavelength of a transmitted wave in the band as adjusted by a value K greater than I corresponding to a phase correction of the actual free space wavelength due to an increased potential velocity established by the size and spacing of said loops, the area enclosed by said loops being between .0227)? and 0038M where A-is said free space wavelength corresponding to said frequency within the transmission band.

2. The antenna according to claim 1, and in which all the loops are alike.

3. The antenna according to claim 1, and in which the loops have circular shapes with diameters between .17)\ and .07 corresponding to a transmitted frequency within the frequency operating band.

4. The antenna according to claim 1, and in which the loops have circular shapes with diameters substantially .115x where A corresponds to a transmitted frequency within the frequency operating band.

5. The antenna according to claim 1, and in which the loops are cophasally energized.

6. The antenna according to claim 1 having a total span in which the loops are contained substantially .81 where A corresponds to a wavelength of a transmitted frequency within the frequency operated band.

7. The antenna according to claim 1 in which the means for energizing the transmission line is connected thereto near one end thereof.

8. In an antenna, a balanced transmission line, a plurality of circular metal loops shunted across said line, each having a diameter substantially .1l57\, x corresponding to a frequency within the transmitted frequency band, the ratio of the space between adjacent loops measured in said wavelength to the said diameter being substantially .895, an unbalanced-to-balanced radio frequency transformer, a concentric transmission line connected to the balanced line near one end thereof through said transformer, the transmeans for attaching the. midpoint of the loops toxthlezmastand means for attaching said other endrofthe-unbalanced transmission line to the mast:

9. In an antenna, a balanced transmission line, a plurality ofrcircular metal loops shunted across said-line,-each having a diameter substantially .115A, A being a wavelength in the transmitted band, the ratio, of the, spacing between adjacent loops tosaid diameter being as small as .895, a concentric transmission line, anunbalanced-tobalanced radio. frequency transformer for conheating the: last mentioned line to the balanced linenear the. middle. point of the latter, movable short-circuiting bars near the ends of the balanced line, a movable short-circuiting bar operatively-connectedwith said transformer for adjusting. the impedance of one line to that of the other, a metal mast, means for attaching the midpoints of the loops to the mast, and means for attaching the ends of the balanced transmission line to the mast.

10. Inv an antenna, a transmission line, a plurality; of more than two loops shunted across said line'and means for energizing said line with ultra high frequency radio waves, said loops being spaced sufficiently close to one another and dimensionedfor such a size whereby said loops are contained in space not more than one-half of an apparent free wave length of the transmitted wave as adjusted by a value K greater than 1 corresponding to a phase correction of the actual free space wave length due to the increased potential velocity established by the size and spacing of said loops, said spacings between loops divided by the loop diameter being substantiallg .895 and the diameter of said loops being not greater than .17 where x corresponds to the transmitted frequency within the frequency operating band.

11.,In an antenna, a transmission line,-a pluralityof morethan two loops shunted across said line andmeans forv energizing said line with ultra high frequency radio waves, said loops being spaced su-fiiciently close to one another and dimensioned. for such a size whereby said loops are contained in space not more than one-half of an apparent free wave length of the transmitted wave as adjusted by a value K greater than 1 corresponding. to a phase correction of the actual free space wave length due to the increased potential velocity established by the size and spacing of said loops, said antenna having a short circuit connector near one end of the transmission line.

12. In an antenna, a transmission line, a plurality of more than two loops shunted across said line and means for energizing said line with ultra high frequency radio waves, said loops being spaced sufiiciently close to one another and dimensioned for such a size whereby said loops are contained in space not more than one-half of anapparent, free wave length of the transmitted wave as adjusted by a value K greater than 1 corresponding toa phase correction of the actual free space wave length due to the increased potential velocity established by the size and spacing of said loops, said means for energizing the transmission linev being connected to the line at its mid-point and said line having short circuit elements near both ends thereof.

13; In an antenna, a transmission:line,- a p111 rality of more than two loops shunted across said line and means for energizing said linewith ultra high frequency radio waves, said loops being spaced sufficiently close to one another and dimensioned for such a size whereby said loops are contained in space not more than one-half of an apparent free wave length of the transmitted wave as adjusted by a value K greater than 1 corresponding to a phase correction of the actual free space wave length due to themcreased potential velocity established by the size and spacing of said loops, said means forenergizing the transmission line being connected thereto near one end and movable short circuiting bars connected near both ends of said line.-

14. In an antenna, transmission line, a plurality of more than two loops shunted acrosssa-id line and means for energizing said line with ultra high frequency radio waves, said loopsv being spaced sufficiently close to one another and di-- potential velocity established by the size and spacing of said loops, said means for energizing the transmission line being connected thereto nearone end and means comprising a coaxial,

transmission line and an unbalanced-to-balanced radio frequency transformer, the coaxial linebeing connected to the transmission line element near one of its ends by the transformer and a short circuiting bar connected across the transmission line near its other end.,

15. In an antenna, a transmission line,.a plu-.

rality of more than two loops shunted across said line and means for energizing said. line with ultra high frequency radio waves, said loops being spaced sufliciently close to one anotherand dimensioned for such a size whereby said loops are contained in space not, more than one-half of an apparent free wave length of the transmitted wave as adjusted by a value K-greater than 1 corresponding to a phase correction of the actual free space wave length due to the increased potential velocity established by the size and spacing of said loops, a metal support to which the loops are attached at their neutral points and means for attaching at least one end of the transmission line to the metal support.

ANDREW ALFORD.

REFERENCES CITED The following references are of record in the? file of this patent:

UNITED STATES PATENTS 

